A femur 1 consists of, as shown in FIG. 26(a), a caput 61, a neck part 62, a greater trochanter 42, and a lesser trochanter (not shown), in the diaphysis 63 thereof a medullary cavity 6 is formed. The surface layer of the femur 1 is cortical bone 66, and in the epiphysis 67 spongiosa 68 supports the cortical bone 66 from the inside by means of back-up effect. The break of the caput 61 engaged with an acetabulum, not shown, of pelvis to form spherical joint, due to a traffic accident or the like as shown in FIG. 26 (b) or deteriorating the femur due to osteoporosis, make it nearly impossible for the femur 1 to support the load 69 transmitted from the pelvis.
It is often possible to recover the function of transmitting the load, depending on the remaining shape of the femur 1 and the stiffness of the cortical bone 66. In order to recover the function, a spherical head 70 working as a caput is fixed to the femur 1 through a stem 71, as shown in FIG. 26(c). Before inserting the stem into the femur, the cortical bone 66 in the epiphysis 67 is generally cut off, as shown by the dot-dash-line in FIG. 26(b). By using a stem 71 and a rasp 46 shown in FIG. 27, the spongiosa 68 is excised so as to form a deep hollow 4. The stem 71 consists of a neck 72 for supporting a spherical head 70, a nose 73 for working as a guide while being inserted into the deep hollow 4 and for keeping the posture thereof after being fixed in the femur, and a body 32 for transmitting the load by shearing stress occurring at the interface between the surface of the stem and the surface of the deep hollow.
So far there are two methods of fixing the stem 71 in the deep hollow 4; a method of uniting the stem with the deep hollow by cement, the other method of filling the interface not with cement but with spongiosa gradually grown on the surface of the deep hollow. Although cement turns into a solid in a short period of time, un-reacted monomer may lixiviate therefrom, often causing the pulmonary embolism. Although Bone-Growth of spongiosa is quite harmless to human bodies, it needs a long hospitalization, for it requires much time to unite them. From that point of view, recently the research and the development of artificial cement-less hip prosthesis stem has been prospering on the grounds that the stem can be easily removed from a femur when replacing the stem in the future and that it will not chemically deteriorate.
The rasp mentioned above is driven into the femur 1 from the epiphysis 67 toward the diaphysis 63, gradually moved up and down to excise the spongiosa 68, thus the deep hollow is formed. In the case of fixing by using cement, an oversize rasp than the stem 71 is used for forming the deep hollow 4, but in the case of fixing by Bone-Growth which makes the stem unite to the femur, an undersize rasp than the stem is used for forming the deep hollow. In any case, the rasp is approximately similar to the stem in shape, but the curvature of the femur 1 and the thickness of the remaining cortical bone 66 differ with patients, stems of ordinary shape and size are often unavailable.
In the case of fixing by using cement, it is not always necessary for a rasp to be closely similar to a stem in shape, because a space is given beforehand for filling cement in the deep hollow 4. Examining the state of wall of the deep hollow in advance from the data of three-dimensional CT graphics of the femur in order to successfully form the deep hollow, may avoid damaging the cortical bone 66 while forming the deep hollow. The stem 71 having little bend as shown in FIG. 26(c) can be inserted straight into the deep hollow previously filled with cement like the arrow 85, which promotes the standardizing the shape and the size of the stem, the handling simply and easily to insert the stem, ready-made of stems, and reducing in doctor's fee, in spite of the drawback that un-reacted monomer may lixiviate from the cement.
In the case of fixing by Bone-Growth, however, a stem and a rasp need to be closely similar in shape and size. The CT graphics data 75 shown in FIG. 28(a) and MRI data are obtained from the femur 1 to have prosthesis, which are peculiar to the patient, by means of non-destroying section photography equipment. The three dimensional graphics 76 shown in FIG. 28 (b) is made from these data, the threshold value of bone density 77, that is Gray Scale, shown in FIG. 28(c) is applied to the graphics, which is necessary for the patient's femur to form the deep hollow. The threshold value determined by the surgeon will detect the limitary thickness of the cortical bone on each part of the femur, the inner surface obtained by smoothing the inside of the bony tissue having the limitary thickness, will assign the suitable shape and the size to the deep hollow.
The idea has already known of custom-making the stem to be implanted in the deep hollow made as mentioned above. But since the stem is a metal product made of titanium alloy or the like which does not usually cause any chemical change and the stem is manufactured by casting, it will cost a great deal to get the stem fitting the patient, including the cost of a metallic mold which has to be made individually for each patient. In JP2001-33792A1, for instance, it is disclosed that a rough part for forming a deep hollow is obtained from many CT graphics of a femur so as to prepare several standards of stem which fit the part. In WO2005/034818, a method is disclosed of making a mold for forming a stem automatically by NC-machine receiving the numerical data indicating the shape of stem, which are equal to a shape of deep hollow computed from CT graphics of the femur to take a surgery according to the surgeon's opinion.
However, designing stem and manufacturing metallic mold by computer have not been realized yet. The actual situation of the process of operation is as follows; the staffs in charge of the operation take CT graphics of the femur and send them to a manufacturer of stem. The manufacturer which has thirty kinds, for the present, of stems of typical shape and size, and the rasps corresponding to the stems on hand, selects five or so stems and rasps nearly agreed with the CT data and sends them to the staffs. The surgeon selects one rasp from them and excises the femur with it so as to find a proper use of it, or may try out other rasps if necessary. While using the rasps, he may find the best rasp which is easy for handling or suitable for the patient to complete the deep hollow.
The stem is used which corresponds to the rasp last-used, the surgeon inserts this stem so as to move along the wall of the deep hollow. Since the deep hollow is slightly smaller than the stem, the stem can be arranged at the desired position by his last pushing or tapping. At this point, some of the stem's surface tightly contact the spongiosa remaining on the surface of the deep hollow, the stem settles in the state that the stem sticks to the femur. The staff in charge decide to purchase the rasp and the stem, sending back other rasps and stems to the manufacturer. The surgeon is obliged to try forming the hollow by using only a few rasps which have been sent from the manufacturer, resulting in that the surgeon cannot select the favorable rasp until he tries all of the rasps.
More particularly, although the surgeon may tell the manufacturer his requirements on the shape and the size of the deep hollow drawn from the CT graphics, the rasps given by the manufacturer are just selected among the standards, accordingly the shape and the size thereof may not always meet with his requirements. Even if a transition in the posture of the rasp is shown by the manufacturer from the beginning of excising the hollow to the end, this is still at the level of a rough suggestion. The surgeon has to start the operation with no opportunity to practice handling the rasp, and does not have much reliance on the rasp throughout the operation. What the surgeon can do is only to do his best in using the rasp sent from the manufacturer, so that it is impossible for him not only to realize his ideal in handling the rasp and the stem in response to the femur of the patient, but to obtain the rasp and the stem to which he introduces his ability and his peculiar way in handling them.
A stem 71 which can be inserted linearly as shown in FIG. 26 (c), is necessarily thinner than the femur 1. A stem without a bend such as the stem 71 has the advantage of being easy to mold, however, in the case that a stem is fixed by Bone-Growth, the stem is liable to be unstable in its posture and to come off from the femur, for the stem is supported by spongiosa 68 whose supporting and fixing properties are not sufficient. Incidentally, a stem made of fiber reinforced plastics is obtainable by using a ceramics mold, so that it should be manufactured at a low price in spite of the fact that it is custom-made, moreover, it is easy to make the stem having a bend in response to the femur of the patient, therefore, it is expected that the stem will be available which fits his femur best.
It is ideal for the stem to be settled in the femur, however, the question is how to successfully bring the stem into the position and the posture. If any part of the stem hits the deep hollow on the surface during the inserting of the stem, further inserting thereof will break the deep hollow, therefore the problem is how to obtain the necessary and sufficient space to move the stem without making the deep hollow oversize than it needs. There are two solutions to the problem, one is that the shape and the size of the stem are determined beforehand and then the suitable shape and size of the deep hollow is obtained which enables the stem to be inserted, and the other is obtained vice versa. In any case, it depends on whether the course can be specified or not that the stem is inserted so as not to hit the inside of the bony tissue having the limitary thickness.
That the stem can be implanted in the fixed preferable position of the deep hollow means that the stem can be removed from the deep hollow. Consequently, it is effective to find out the course that the stem settled in the deep hollow is successfully removed from the deep hollow. As illustrated in FIG. 29, the shadowed stem 80 in fixed position is lifted by one step as shown by the arrow 81, successively lifted step by step through the position of the stem 82, removed from the deep hollow at the position of the stem 83.
Even in the first step, at least four kinds of positions and postures of the stem 84 are shown in Figure, where the stem is lifted in the deep hollow 4 by a height of the arrow 81. It goes without saying that there are boundless positions and postures theoretically in spite of the fact that the stem is restricted to move in the deep hollow. Moving up the stem to the next step of the arrow 85, makes the number of postures of the stem multiply boundlessly every posture of the last step, not shown. Furthermore, much more numbers of the positions and postures of the stem at the second step have to be assumed than those of the first step, thus there are limitless courses to reach the stem 83 of the last step. Enormous calculations will be needed before preparing for detecting the preferable course, resulting in mere waste of time and money. It is absolutely impossible to find out the best combination of the stem and the deep hollow for taking the positions and the postures to be adopted from the boundless combinations, with grasping the rates of Fit and Fill of each stem.
This problem is caused by that the deep hollow has extra room for the stem to move therein. However, if the deep hollow is too tight to restrict the movement of the stem, the stem cannot be lifted from the starting position or perhaps on the way, making it impossible to calculate. This is because the stem whose curvature varies with its part cannot be removed from the deep hollow when the shape and the size of the deep hollow are the same or almost the same as those of the stem. In determining the shape of the stem which can be inserted into the deep hollow, making the deep hollow beforehand to be oversize than the stem causes the limitless kinds of postures of the stem, which makes it impossible to calculate, as mentioned above.
Patent Document 1 JP2001-33792A1
Patent Document 2 WO2005/034818